These results imply that the superconductivity in FeSe is not simply described by a picture of the conventional BCS-BEC crossover. An important aspect which has not been taken into account in the previous studies is the effects of multiband electronic structure, which may have other tuning factors of the crossover in addition to \(\Delta /\varepsilon _{\rm F}\) . In fact, Fermi surface of FeSe is composed of separated hole and electron pockets with different values of \(\Delta /\varepsilon _{\rm F}\)  [1]}, [2]}, [3]}. It has been pointed out that in the multiband systems the developments of superconducting fluctuations and pseudogap formation are sensitively affected by the interband coupling strength [4]}, indicating that the normal and superconducting properties associated with the crossover in multiband systems can be different from those in the single band systems [5]}. It is quite important, therefore, to clarify how the superconducting properties evolve when the multiband electronic structure is tuned.
